How to swizzle a method of a private class

Question

I have a private class (both declared & defined within .m) as an addition to an implementation of a different class, that happens to use that private class internally.

I'd like to swizzle one of the methods of that private class.

I defined a category and did the usual:

+(void)load 
{
    Method original, swizzled;

    original = class_getInstanceMethod(objc_getClass("SomePrivateClass"), @selector(somePrivateMethod:));
    swizzled = class_getInstanceMethod(self, @selector(swizzled_somePrivateMethod:));
    method_exchangeImplementations(original, swizzled); 
}

The issue is that my implementation obviously doesn't know anything about this private class and self refers to the class I am adding the category to, whichever class that might be. So I have no way of calling the original implementation and in general working with the private class.

What is the proper approach to tackle this?

Answer 1

Managed to get this to work, it's pretty simple actually.

So the way I did it:

• made a NSObject category: @interface NSObject(PrivateSwizzleCategory)

• swizzled:

  +(void)load
  {
      Method original, swizzled;
  
      original = class_getInstanceMethod(objc_getClass("SomePrivateClass"), @selector(somePrivateMethod:));
      swizzled = class_getInstanceMethod(self, @selector(swizzled_somePrivateMethod:));   
      method_exchangeImplementations(original, swizzled);
  }
  

• To call the original implementation, I had to cast self to NSObject:

  id ret = [(NSObject *)self swizzled_somePrivateMethod:someParam];
  

• To access private properties of the private class, I used valueForKey on self:

  id privateProperty = [self valueForKey:@"__privateProperty"];
  

Everything works!